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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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476                                ALASKAN GLACIER STUDIES
At the lighthouse east of Einchinbrook Entrance the glaciated ledges 185 feet above sea level bear striae which trend nearly due east. It is clear that these glacial scratches have been made by ice which came from Hinehinbrook Entrance of Prince William Sound, rather than from Orca Inlet to the northeast.
The granite bowlders in the glacial till here and the striations show, not only that the Prince William Sound Glacier extended out into the Pacific Ocean, but that it spread out in a bulb as soon as it was released from the confining walls of Hinchinbrook Entrance. This explains the eastward trend of the striae at Cape Hinchinbrook, in contrast with the southward trend in Hiuchinbrook Entrance, and suggests that the Pacific side of Montague Island may have similar striae. A search for striae on Seal Rocks, which rise to a height of thirty-seven feet off the mouth of Hinchinbrook Entrance would be of great interest, both as to whether the Prince William Sound Glacier extended this far into the Pacific Ocean, and, if so, as to the direction of the ice movement here.
The easternmost outlet of Prince William Sound lies between Hinchinbrook and Hawkins Islands near the mouth of Orca Inlet. This outlet seems to have been of decidedly minor importance, so little ice moving through it that it was not eroded below sea level, excepting in the narrow Hawkins Island Cut-off and the still smaller Canoe Passage. There was, however, strong ice movement out of the northeastern portion of Orca Inlet.
The Driftless Area on Hinchinbrook Island. At the time of maximum glaciation, the Prince William Sound Glacier did not completely cover Hinchinbrook Island, which rose through it as a nunatak. The glacial deposits, at least in the southwestern portion, are entirely confined to its lower slopes, a large part of the island forming a driftless-area.
This was determined in 1910 by observations at two points near the southwestern end of the island. Between English Bay, a cove on the southern side of Port Etches, and the top of Signal Mountain, a painstaking examination revealed glacial deposits only on the lower slopes. The till contained bowlders of granite and other foreign material not present in the country rock of Hinchinbrook Island, showing that the ice which carried this glacial drift was not a local glacier but the great ice sheet from northern Prince William Sound. The glacial deposits were found upon the gentler lower slopes, with a thickness of 15 or 20 feet, as on the headland southwest of English Bay; and a steepened slope, extending up to an elevation of less than 400 feet, showed the maximum thickness-of the glacier. At about 400 feet there is also a small hanging valley east of English Bay.
At an elevation of 440 feet glacial deposits were entirely lacking, as we found by excavating through the thick peaty soil and residual deposits to bedrock. Frequent ledges from this point to the top of Signal Mountain revealed absolutely no rounding or stria-tion, and there were no rounded pebbles excepting those traceable to certain, visibly-disintegrating, conglomerate ledges. The broad, rounded top of Signal Mountain, 1646 feet high, showed clear evidence of never having been glaciated, a careful examination revealing no erratics, no striae, and no rounded pebbles except those from the weathered ledges of conglomerate.
The second set of observations as to the height of glaciation was made at Cape Hinchinbrook on the Pacific side of the island near the new lighthouse on the southwestern tip. The lowest slopes were without glacial deposits, because they had been cut by the waves into great, precipitous seacliffs. Back of the cliffs, however, glacial till was ob-